Ultrastrong coupling of SiGe parabolic quantum wells to terahertz patch antennas

Control and manipulation of quantum states by light are increasingly important for both fundamental research and applications. This can be achieved, among other techniques, through the strong coupling between light and semiconductor quantum wells, typically observed at THz frequencies in integrated lithographic optical cavities. Here, we explore the possibility of achieving ultrastrong coupling at room temperature between conduction sub-band states in Si(1-x)Gex heterostructures and THz cavity photons fabricated with a potentially silicon-CMOS-compliant process. We developed Si(1-x)Gex parabolic quantum wells in the conduction band (0.8 < x < 1.0) with temperature-independent transitions in the 3 to 4 THz range, embedded in hybrid metal-plasmonic THz patch-antenna microcavities resonating between 2 and 7 THz depending on the square patch length. In this first demonstration, we achieved anticrossing around 3.0 THz and around 3.8 THz with spectroscopically measured Rabi frequency of 0.7 THz, leading to an ultrastrong coupling regime where the ratio between Rabi and resonant frequency surpasses 0.2.